A guide to key-value methods

I have a second draft for TUGBoat on the go at the moment: an article on implementing key-value input. These are widely used in LaTeX packages, but are also usful in plain TeX. As well as the original keyval package, we have xkeyval, kvoptions and kvsetkeys for managing key–value input. However, actually getting started with these as a LaTeX programmer can be somewhat difficult. The aim of the article is to lower the barrier to getting going with key-value methods. As well as the keyval-based information, the pgfkeys package (part of the pgf bundle) is also covered (thanks to Christian Feuersänger).